Alternators for generating electrical power typically include an electromagnet for generating a field of magnetic flux. The output voltage of the alternator is commonly regulated by varying the current flow through this electromagnet and thereby varying the field flux. Permanent magnet alternators, on the other hand, have a relatively fixed field flux supplied by permanent magnets. One method of regulating the output voltage of the alternator is to mechanically move either the permanent magnets or some other member that affects the flux that the moving coil encounters during operation. One such device is disclosed in U.S. Pat. No. 4,766,362 which issued Aug. 23, 1988 to Sadvary. This permanent magnet alternator utilized a magnetic flux modulating sleeve that is carried by and rotates with the rotor. The physical position of the sleeve with respect to the rotor is changeable during rotor rotation to vary the flux field and provide a constant voltage output during load and rotor speed variations. Such mechanical systems of control, however, are not sufficiently responsive to changing load and speed conditions found in some applications. Additionally, such mechanical systems are relatively expensive to manufacture, are prone to wear and tear, and require significant maintenance to remain operational. Another method of regulating the output voltage of the alternator is to provide a separate wound field that can have a forward polarity current applied for enhancing the affect of the flux field of the permanent magnets during low speed rotation of the alternator rotor and a reverse polarity current to offset the affect of the flux field during high speed rotation. Such a voltage regulator system is disclosed in U.S. Pat. No. 5,502,368 which issued Mar. 26, 1996 to Syverson et al. An additional method of regulating the output voltage of a permanent magnet alternator is disclosed in U.S. Pat. No. 4,659,978 which issued Apr. 21, 1987 to Dogadko. The regulator of the '978 patent utilizes a pair of silicon controlled rectifiers (SCR) to shunt the output current of the alternator to ground when the output voltage exceeds a desired value. This method requires a somewhat complex circuit structure that can be expensive to manufacture. Another method of regulating the output voltage of a permanent magnet alternator is to apply a selected load to the output thereby causing the voltage to vary in correspondence to the applied load. This is sometimes accomplished by simply a fixed resistance, or by a more sophisticated circuit that senses the amount of change in load needed to effect a desired change in output voltage and then applying that precise amount of load. In any case, the application of an additional load to the alternator causes substantial heat that must be dissipated and wastes energy that could otherwise be utilized.
What is needed is a voltage regulator for a permanent magnet alternator that is effective during a wide range of rotor speed and load variations, and that requires little or no maintenance. Additionally, the regulator should be relatively simple and inexpensive to manufacture and should not generate substantial unnecessary additional heat or waste energy.